The question remains controversial as to whether homosexuality is a personal choice, a result of environmental influences, biological or even hard-wired through genes — or some combination of these.

Over the years, it has been considered a sin, a crime, a mental illness and, since 1973 by the American Psychiatric Association, a variant of normal human sexual behavior

Among the research:

By the 1980s: Scientists agreed adult hormone levels were not a factor and began examining hormone exposure in the womb and genetic influences, so far without conclusive results.

1991: A study of identical twins by Northwestern psychologist Michael Bailey found that if one was gay, the other had a 52 percent chance of being gay. The findings indicate heredity plays a role in homosexuality but is not the only factor.

1993: Geneticist Dean Hamer reported in Science that he had pinpointed a section of chromosome Xq28 as being involved in male homosexuality.

1996: University of Toronto psychiatric researcher Ray Blanchard found that men with older brothers were more likely to be gay than those without. A similar study was made in 2006.

2007: Stanton Jones, a psychology professor at Christian Wheaton College, led a study that found 67 percent of 98 participants seeking to change from homosexual to heterosexual were successful using religious mediation.

2008: A study in the Netherlands reported that sexual differentiation of the human brain occurs during fetal and newborn development and programs gender identity.